Method and apparatus for etching photomasks

ABSTRACT

A process is provided for etching a silicon based material in a substrate, such as a photomask, to form features with straight sidewalls, flat bottoms, and high profile angles between the sidewalls and bottom, and minimizing the formation of polymer deposits on the substrate. In the etching process, the substrate is positioned in a processing chamber, a processing gas comprising a fluorocarbon, which advantageously is a hydrogen free fluorocarbon, is introduced into the processing chamber, wherein the substrate is maintained at a reduced temperature, and the processing gas is excited into a plasma state at a reduced power level to etch the silicon based material of the substrate. The processing gas may further comprise an inert gas, such as argon.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of co-pending U.S. patentapplication Ser. No. 10/126,851, filed on Apr. 19, 2002, which is acontinuation of U.S. patent application Ser. No. 09/625,343, filed onJul. 25, 2000, that claims priority to U.S. Provisional PatentApplication Ser. No. 60/206,230, filed on May 22, 2000. Each of therelated applications is incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the fabrication of integrated circuits and tothe fabrication of photomasks useful in the manufacture of integratedcircuits.

2. Background of the Related Art

Semiconductor device geometries have dramatically decreased in sizesince such devices were first introduced several decades ago. Sincethen, integrated circuits have generally followed the two year/half-sizerule (often called Moore's Law), which means that the number of deviceson a chip doubles every two years. Today's fabrication plants areroutinely producing devices having 0.35 μm and even 0.18 μm featuresizes, and tomorrow's plants soon will be producing devices having evensmaller geometries.

The increasing circuit densities have placed additional demands onprocesses used to fabricate semiconductor devices. For example, ascircuit densities increase, the widths of vias, contacts and otherfeatures, as well as the dielectric materials between them, decrease tosub-micron dimensions, whereas the thickness of the dielectric layersremains substantially constant, with the result that the aspect ratiosfor the features, i.e., their height divided by width, increases.Reliable formation of high aspect ratio features is important to thesuccess of sub-micron technology and to the continued effort to increasecircuit density and quality of individual substrates and die. Highaspect ratio features are conventionally formed by patterning a surfaceof a substrate to define the dimensions of the features and then etchingthe substrate to remove the substrate material. Consequently, reliableformation of high aspect ratio features requires a precise patterningand etching of the substrate.

Photolithography is a technique used to form precise patterns onsubstrates to be etched to form the desired devices or features.Generally, photolithography techniques use light patterns to exposephotoresist materials deposited on a substrate surface to developprecise patterns on the substrate surface prior to the etching process.In conventional photolithographic processes, a photoresist is applied onthe material to be etched, and the features to be etched in thematerial, such as contacts, vias, or interconnects, are defined byexposing the photoresist to a pattern of light through aphotolithographic photomask which corresponds to the desiredconfiguration of features. A light source emitting ultraviolet (UV)light, for example, may be used to expose the photoresist to chemicallyalter the composition of the photoresist. The altered or the unalteredphotoresist material is then removed by chemical processes to expose theunderlying material of the substrate while the retained photoresistmaterial remains as a protective coating. Once the desired photoresistmaterial s removed to form the desired pattern in the photoresist, theexposed underlying material is then etched to form the features in thesubstrate surface.

Photolithographic photomasks, or reticles, typically include a substratemade of an optically transparent silicon based material, such as quartz(i.e., silicon dioxide, SiO₂), having an opaque light-shielding layer ofmetal, typically chromium, patterned on the surface of the substrate.The metal layer is patterned to form features which define the patternand correspond to the dimensions of the features to be transferred tothe substrate. Generally, conventional photomasks are fabricated byfirst depositing a thin layer of metal on a substrate comprising anoptically transparent silicon based material, such as quartz, anddepositing a photoresist layer on the thin metal layer. The photoresistis then patterned using conventional patterning techniques. The metallayer is etched to remove material not protected by the photoresist,thereby exposing the underlying silicon based material.

In order to achieve current circuit densities, alternating phase shiftphotomasks are being used to increase the precision of the etchingpattern formed on the substrate by increasing the resolution of thelight passing through the photomask. Alternating phase shift photomasksare fabricated by the same method described above, but with theadditional step of etching the exposed silicon based material to formfeatures that refract the light passing therethrough by onehalf-wavelength. The half-wavelength light has a greater intensity andimproved resolution over the unmodified light, thereby allowing theformation of more precise patterns on the underlying substrate. Therefraction of light to produce a proportionally shortened wavelength isbased on the composition and thickness of the substrate, and thephotomask features are etched into the silicon based material to changethe thickness of the material the light passes through, and thus changethe wavelength of the light. To modify the light to produce the desiredwavelength, the etched features formed in the silicon based material ofthe substrate must be precisely formed in the substrate with a minimalamount of defects in the feature structure.

Current etching processes for silicon based materials, such as thosematerials used for dielectric layers in semi-conductor manufacturing,have proven unsuitable for etching features in photomasks. For example,the required processing temperatures, or thermal budgets, of materials,such as photoresists, used in photomask fabrication, are lower than thetemperatures experienced in conventional dielectric etching processes.If the thermal budget is exceeded during etching of the photomask, thephotoresist layer can detrimentally deteriorate, and consequently causeimprecise features to be etched in the underlying silicon basedmaterial, resulting in the formation of defective photomasks.

Additionally, current etch chemistries, such as a mixture of CHF₃ andoxygen, used to etch silicon based substrates in photomask fabricationhave not produced quality photomasks because the chemistry and theprocessing conditions have not been able to achieve acceptable featurestructure. High quality photomasks require features etched in thesilicon based material to have straight sidewalls, a flat bottom, and aangle between the sidewalls and the bottom of the feature, which isreferred to as a profile angle, between about 85° and about 90°. If theprofile angle is formed with unacceptable tolerances, i.e., angles ofless than about 85°, the properties of the light passing through thefeature may be detrimentally affected, such as having a less thandesirable light resolution, and produce less than desired patterning ofthe underlying substrate.

One difficulty with achieving acceptable feature structure by currentetch chemistries and processing conditions occurs when the CHF₃processing gas produces plasma radicals, such as CHF₂, which canpolymerize and form deposits on the surfaces of the features formed inthe silicon based material of the photomask or on the processingchambers surfaces. The polymer deposits may then flake and produce aparticle problem in the chamber and in the etched features. Particledeposition in the features can interfere with the etching process andresult in imprecisely formed features. Particle deposition in thefeatures after etching can also lead to interference with the lightpassing therethrough to produce numerous patterning defects in thesubsequent photolithograpic processing of substrates.

Polymer deposits may also form on the inner surfaces of the features,and prevent consistent etching of the features, particularly on thebottom and lower sidewalls of high aspect ratio features. In order toetch the silicon based material of the substrate, the etch process firstremoves any polymer deposits formed thereon prior to etching theunderlying silicon based material. The etching interference caused bythe deposited polymers, or passivating deposits, can result in featuresformed with undesirable structures. For example, it has been observedthat the current etch chemistries and processing conditions for etchingsilicon base materials produce features with uneven or taperingsidewalls, feature bottoms which are convex, concave, or have roughsurfaces, and profile angles of less than 85°. Such featuresdetrimentally affect the light passing therethrough which can result inimprecise patterning by the photomask.

The polymer deposits may also detrimentally affect the etching rate ofthe silicon based materials in comparison to the etching rate of thephotoresist materials to produce imprecisely formed features. Theetching rate, or removal rate, of one material in contrast to anothermaterial is often described as the selectivity of the process to thematerials. Polymer deposits formed on the surfaces of the siliconmaterial are etched from the surface of the features prior to theunderlying silicon, thereby resulting in the silicon material beingetched at a slower rate than would normally occur. A lower etching rateof the silicon material having polymer deposits formed thereon incomparison to the photoresist material etch rate corresponds to a lowerselectivity. The effect of a lower selectivity is the premature removalof photoresist material which may produce features which are not etchedto the desired dimensions, or which may have tapered dimensions orrounded corners at the top and bottom of the feature. The impreciselyformed features can detrimentally affect the resolution and opticalperformance of light passing therethrough.

Therefore, there remains a need for a photomask etching chemistry andprocess which limits polymer formation, minimizes defect formation, andforms features with straight sidewalls, flat, even bottoms, and highprofile angles. It would also be desirable if the process providedimproved etch selectivity.

SUMMARY OF THE INVENTION

The invention generally provides a method for etching a substrate, suchas a photomask comprising a silicon based material. In one aspect, amethod is provided for etching a substrate comprising placing thesubstrate on a support member in a processing chamber where thesubstrate comprises a silicon based material, introducing a processinggas comprising one or more hydrogen free fluorocarbons into theprocessing chamber, delivering power to the processing chamber togenerate a plasma, and etching exposed portions of the silicon basedmaterial. The processing gas may comprise an inert gas, a polymerizationlimiting gas, and combinations thereof.

In another aspect, a method is provided for etching a substratecomprising quartz, a patterned metal layer deposited on the quartz, anda patterned resist material deposited on the patterned metal layer. Themethod comprises placing the substrate on a support member in aprocessing chamber, introducing a processing gas comprising one or morefluorocarbon gases into the processing chamber, delivering power to theprocessing chamber by supplying a source RF power of about 500 Watts orless to a coil and supplying a bias power to the support member of about500 Watts or less to generate a plasma, and etching exposed portions ofthe quartz. The processing gas may comprise an inert gas, apolymerization limiting gas, and combinations thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features, advantages andobjects of the invention are attained and can be understood in detail, amore particular description of the invention, briefly summarized above,may be had by reference to the embodiments thereof which are illustratedin the appended drawings.

It is to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a schematic view of an exemplary etching chamber for use withthe processes described herein;

FIG. 2 is a flow chart illustrating a sequence for processing asubstrate according to one embodiment of the invention;

FIGS. 3A-3E are cross sectional views showing an etching sequence of oneembodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention will be described below in reference to an inductivelycoupled plasma etch chamber, such as a Decoupled Plasma Source, or DPS™,chamber manufactured by Applied Materials, Inc., of Santa Clara, Calif.Other process chambers may be used to perform the processes of theinvention, including, for example, capacitively coupled parallel platechambers and magnetically enhanced ion etch chambers as well asinductively coupled plasma etch chambers of different designs. Althoughthe processes are advantageously performed with the DPS™ processingchamber, the description in conjunction with the DPS™ processing chamberis illustrative, and should not be used to limit the scope of theinvention.

FIG. 1 is a schematic cross sectional view of one embodiment of a DPS™processing chamber which may be used for performing the processesdescribed herein. The processing chamber 10 generally includes acylindrical sidewall or chamber body 12, an energy transparent domeceiling 13 mounted on the body 12, and a chamber bottom 17. An inductivecoil 26 is disposed around at least a portion of the dome 13. Thechamber body 12 and chamber bottom 17 of the processing chamber 10 canbe made of a metal, such as anodized aluminum, and the dome 13 can bemade of an energy transparent material such as a ceramic or otherdielectric material. A substrate support member 16 is disposed in theprocessing chamber 10 to support a substrate 20 during processing. Thesupport member 16 may by a conventional mechanical or electrostaticchuck with at least a portion of the support member 16 beingelectrically conductive and capable of serving as a process biascathode. A plasma zone 14 is defined by the process chamber 10, thesubstrate support member 16 and the dome 13.

Processing gases are introduced into the processing chamber 10 through agas distributor 22 peripherally disposed about the support member 16. Aplasma is formed from the processing gases using a coil power supply 27which supplies power to the inductor coil 26 to generate anelectromagnetic field in the plasma zone 14. The support member 16includes an electrode disposed therein which is powered by an electrodepower supply 28 and generates a capacitive electric field in theprocessing chamber 10. Typically, RF power is applied to the electrodein the support member 16 while the body 12 is electrically grounded. Thecapacitive electric field is transverse to the plane of the supportmember 16, and influences the directionality of charged species morenormal to the substrate 20 to provide more vertically orientedanisotropic etching of the substrate 20.

Process gases and etchant byproducts are exhausted from the processchamber 10 through an exhaust system 30. The exhaust system 30 may bedisposed in the bottom 17 of the processing chamber 10 or may bedisposed in the body 12 of the processing chamber 10 for removal ofprocessing gases. A throttle valve 32 is provided in an exhaust port 34for controlling the pressure in the processing chamber 10. An opticalendpoint measurement device can be connected to the processing chamber10 to determine the endpoint of a process performed in the chamber.

Exemplary Etch Process

A silicon based substrate, such as quartz, used in manufacturingphotomasks is etched to produce features having straight verticalsidewall definition with sharp angled profiles and a flat featurebottom. A processing gas comprising fluorocarbon gases are used.Advantageously, the fluorocarbon gases are hydrogen free fluorocarbons.The fluorocarbons can have from 1 to 5 atoms of carbon and from 4 to 8atoms of fluorine is used to reduce the formation of polymer deposits onthe substrate surface. Examples of hydrogen free fluorocarbon gasesinclude CF₄, C₂F₆, C₄F₆, C₃F₈, C₄F₈, C₅F₈, and combinations thereof.

The processing gas may also include an inert gas which when ionized aspart of the plasma comprising the processing gas results in sputteringspecies to increase the etching rate of the features. The presence of aninert gas as part of the plasma may also enhance dissociation of theprocessing gas. Examples of inert gases include argon, helium, neon,xenon, and krypton, and combinations thereof.

The processing gas may also include a polymerization limiting gas thatmay be used to control the amount of polymerization of the hydrogen freefluorocarbon gas. Examples of polymerization limiting gases includeoxygen containing compounds, such as oxygen, and nitrogen containingcompounds, such as nitrogen, and combinations thereof. Examples ofpolymerization limiting gases include oxygen containing gases, such asoxygen (O₂), ozone (O₃), various forms of nitrogen oxide compounds(N_(X)O_(Y)) including nitrous oxide (N₂O), and combinations thereof.Nitrogen containing compounds that may react as polymerization limitinggases including ammonia and NF₃ may also be used. The use of theprocessing gas in the etching process will now be described.

In one embodiment, the substrate is maintained at a temperature betweenabout 50° C. and about 150° C. during etching of the substrate. Heatdegradation of resist materials, such as photoresist materials,deposited on the substrate during the photomask fabrication process isminimized. While the following process is described using photoresistmaterials, other resist materials, such as e-beam resist and X-rayresist materials, among other, are contemplated by the invention herein.It is also believed that the substrate temperature also minimizespolymer formation on the substrate during the etching process.Additionally, the sidewalls of the processing chamber are maintained ata temperature of less than about 70° C. and the dome is maintained at atemperature of less than about 80° C. to maintain consistent processingconditions and to minimize polymer formation on the surfaces of theprocessing chamber.

A source RF power level of about 500 watts or less, for example, betweenabout 50 watts and about 250 watts or between about 50 watts and about200 watts, is applied to an inductor coil to generate and sustain theplasma of the processing gases during the etching process. The source RFpower level produces sufficient radicals from the processing gases toetch the exposed silicon based material of the substrate while providinga sufficiently low power level to minimize the formation of polymerradicals formed in the plasma during the etching process. A bias powerof about 500 watts or less, for example, between about 50 watts andabout 250 watts or between about 50 watts and about 200 watts, is alsoapplied to the substrate support to enhance control of the etchingprocess by providing increased directionality of the etching radicalswith respect to the surface of the substrate.

Further, the power levels used during the etching process aresufficiently low compared to prior art silicon etch processes tomaintain the substrate at temperatures between about 50° C. and about150° C. It is believed that generating a plasma of the processing gasesat reduced power levels and reduced substrate temperatures removesexposed silicon based material and prevents photoresist degradation toproduce features in the silicon based material having straight sidewallsand flat bottoms, with the sidewalls of the etched features formingprofile angles of at least about 85° with the bottom surface of thefeature. It is also believed that the power levels provide improvedselectivity and substantially maintains the critical dimensions of thefeatures being etched.

FIG. 2 is a flow chart of one embodiment of one process sequence of theinvention. The flow chart is provided for illustrative purposes andshould not be construed as limiting the scope of the invention. Asubstrate, typically comprising a silicon based material, such asoptical quality quartz or molybdenum silicide, is provided 200 to aprocessing chamber, such as the DPS™ processing chamber 10 of FIG. 1.The substrate is then processed by depositing an opaque, conformal metallayer, typically chromium, on the substrate 220. The dimensions offeatures to be formed in the metal layer are patterned by depositing andpattern etching a first photoresist material 230 to expose the conformalmetal layer. The photoresist materials used in photomask fabrication areusually low temperature photomask materials, which is defined herein asa photomask material that thermally degrades at a temperature aboveabout 250° C.

Features are then formed in the substrate by etching the conformal metallayer 240 to expose the underlying substrate. The photoresist materialremaining after etching the conformal metal layer is usually thenremoved. The silicon based material of the substrate is prepared foretching by depositing and pattern etching a second photoresist material250 to expose the substrate. The substrate is then transferred to a DPS™processing chamber where a processing gas containing fluorocarbon gasesis introduced into the processing chamber and a plasma is generated,thereby etching 260 the exposed silicon based material of the substrate.

FIGS. 3A-3E illustrate the composition of the photomask prior to theetching steps as well as further illustrate the process described abovein FIG. 2. A substrate 300 made of optical quality quartz material 310is introduced into a processing chamber and a metal layer 320 made ofchromium is deposited thereon as shown in FIG. 3A. The chromium layermay be deposited by conventional methods known in the art, such as byphysical vapor deposition (PVD) or chemical vapor deposition (CVD)techniques. The chromium layer 320 is typically deposited to a depth ofapproximately 200 nanometers (nm) thick, however, the depth of the layermay change based upon the requirements of the manufacturer and thecomposition of the materials of the substrate or metal layer. Referringto FIG. 3B, the substrate 300 is then transferred to another processingchamber where a layer of photoresist material 330, such as “RISTON,”manufactured by duPont de Nemours Chemical Company, is deposited uponthe chromium layer 320. The resist material 330 is then pattern etchedusing conventional laser or electron beam patterning equipment to formfeatures 325 which are used to define the dimensions of the features tobe formed in the chromium layer 320.

The substrate 300 is then transferred to an etch chamber and thechromium layer 320 is etched using metal etching techniques known in theart or by new metal etching techniques that may be developed to formfeatures 335 which expose the underlying quartz material 310 as shown inFIG. 3C. After etching of the chromium layer 320 is completed, thesubstrate 300 is transferred to a processing chamber, and the remainingphotoresist material 330 is usually removed from the substrate 300, suchas by an oxygen plasma process, or other resist removal technique knownin the art. A second photoresist material 340 is then applied andpatterned to expose the underlying quartz material 310 within thefeatures 335 as shown in FIG. 3C. The photoresist material 340 isdeposited to a depth of about 200 nm thick, but may be of any thicknessand is preferably of at least the same thickness as the depth of thefeatures to be etched in the quartz material 310 to form the photomask.The patterned substrate 300 is then transferred to an etch chamber, suchas the DPS™ processing chamber 10, for plasma etching the quartzmaterial 310.

Once placed on the support member 16, a processing gas is introducedinto the chamber and a plasma is generated to etch the quartz material310 of the substrate 300. Advantageously, the processing gas includesfluorocarbon gases of hydrogen free fluorocarbons including CF₄, C₂F₆,C₄F₆, C₃F₈, C₄F₈, C₅F₈, and combinations thereof. A plasma of thefluorocarbon gases produces fluorine-containing species that etch thequartz material 310 on the substrate 300 without the presence of anoxidizing gas. The fluorine containing species of the plasma react withthe quartz material 310 to form volatile SiF_(x) species that areexhausted from the processing chamber 10. It has also been determinedthat the fluorocarbon gases etch the silicon material at an etching rategreater than an etching rate of resist materials.

The processing gas may also include an inert gas including argon,helium, neon, xenon, and krypton, and combinations thereof to improvethe etching process. The processing gas may also include oxygen,nitrogen, or other polymerization limiting or inhibiting gas to improvethe etching results.

The fluorocarbon gases, and advantageously the hydrogen freefluorocarbons, have been observed to be well suited for etching othersilicon based materials, such as molybdenum silicide (MoSi) andmolybdenum silicon oxynitride (MoSiON), used in photomask manufacturing.Further, it is contemplated that the etch chemistry and the processingconditions may also be used to etch dielectric layers containingsilicon, such as silicon oxide, titanium silicide, and silicon nitride,as well as other silicon based materials, such as undoped silicateglass, phosphosilicate glass, and borophosphatesilicate glass, which areused in semiconductor manufacturing.

An exemplary processing regime for etching substrates with a processinggas comprising fluorocarbon gases is as follows. The processing gascomprising the fluorocarbon gases is introduced into a processingchamber, such as the DPS™ described above, at a flow rate between about25 sccm and about 100 sccm. In one embodiment, processing gas isintroduced into a processing chamber at a flow rate between about 25sccm and about 50 sccm. The chamber is maintained at a pressure betweenabout 2 milliTorr and about 50 milliTorr. In one embodiment, thepressure is maintained between about 2 milliTorr and about 10 milliTorr.In another embodiment, the pressure is maintained between about 2milliTorr and about 6 milliTorr. If an inert gas is used to enhance theetching process, the inert gas is introduced into the processing chamberat a flow rate between about 30 sccm and about 150 sccm. In oneembodiment, the inert gas is introduced into the processing chamber at aflow rate of about 50 sccm. A polymerization limiting gas may beintroduced into the processing chamber at a flow rate between about 50sccm or less, preferably about 20 sccm or less. For examplepolymerization limiting gas is introduced into the processing chamber ata flow rate between about 1 sccm and about 10 sccm, such as betweenabout 2 sccm and about 7 sccm, including 5 sccm.

A source RF power of about 500 watts or less, for example, between about50 watts and about 250 watts, is applied to an inductor coil to generateand sustain the plasma during the process. In one embodiment, a sourceRF power level of between about 50 watts and about 110 watts is appliedto the inductor coil. A bias power level of about 500 watts or less, forexample, between about 50 watts and about 250 watts, is applied to thesubstrate support to enhance control of the etching process. In oneembodiment, a bias power level of between about 100 watts and about 200watts is applied to the substrate support.

During the etching process, the substrate is maintained at a temperaturebetween about 50° C. and about 150° C. Additionally, the sidewalls 15 ofthe processing chamber 10 are maintained at a temperature of less thanabout 70° C. and the dome is maintained at a temperature of less thanabout 80° C. to maintain consistent processing conditions and tominimize polymer formation on the surfaces of the processing chamber.Under the above described processing regime parameters, the quartzmaterial 310, or other silicon based materials, can be etched at a ratebetween about 200 Å/min and about 1000 Å/min depending on thecomposition of the processing gas and construction of the processingchamber.

In one embodiment of the invention, a processing gas comprising C₂F₆ isintroduced into a processing chamber at a flow rate of about 25 sccm andthe processing chamber is maintained at a pressure of about 6 Torr. Asource RF power of about 100 watts is applied to an inductor coil togenerate and sustain the plasma during the process with a bias power ofabout 200 watts applied to the substrate support to enhance control ofthe etching process. The substrate is maintained at a temperaturebetween about 50° C. and about 80° C. with the sidewalls of theprocessing chamber maintained at a temperature of about 70° C. and thedome is maintained at a temperature of about 80° C.

Referring back to FIG. 3D, the above described processing regime willetch the quartz material 310 to define the alternating phase shiftfeatures 345 of the photomask. The alternating phase shift features 345formed by this process have straight sidewalls, flat, even bottoms, andhigh profile angles. Once the etching of the quartz material 310 isfinished, the remaining resist material 340 surrounding the photomaskfeatures 355 is removed, such as by an oxygen plasma or other resistremoval technique known in the art, as shown in FIG. 3E.

Precision in etching silicon based materials, such as quartz, requirescontrolling and minimizing the formation of polymer deposits withinsubstrate features. One approach to achieve this precision is to controlthe etch process by using the hydrogen free fluorocarbon gases asdescribed above which minimize the formation of carbon based polymerdeposits that can interfere with the etching process. The reducedpresence of passivating deposits during the etching process achievesfeatures formed with relatively straight side walls, flat bottoms, andprofile angles of greater than 85°, and preferably greater than 88°.

Polymer formation can be minimized by controlling the amount of fluorineand carbon radicals produced during the etching process since carbon andfluorine radicals have been observed to have different effects on theetching process. For example, fluorine radicals are highly reactive andincrease the etching rate of the silicon and silicon based materials byforming volatile SiF_(x) radicals. Carbon radicals, such as the CF₂radical, tend to polymerize and form deposits on the surfaces of thefeatures during the etching process, which deposits can interfere in theetching process and form defects in the etched feature.

It is believed that minimization of the formation of hydrogen containingcarbon radicals, for example CHF from CHF₃, reduces the presence ofreadily available polymerizable species in the plasma. Hydrogencontaining carbon radicals have a greater tendency than hydrogen freecarbon radicals to form deposits on the surface of a substrate.Additionally, polymer deposits formed from hydrogen containing carbonradicals are less reactive than hydrogen free deposits and are moredifficult to remove from the surface of a substrate by traditionaletching processes. Moreover, it is believed that avoiding the formationof hydrogen containing polymer deposits results in an etch rate of thesilicon based material greater than the etch rate of the patternedphotoresist material. Further, it has been observed that gases such asoxygen and nitrogen limit polymerization of the etching gases and allowimproved control over the amount of polymerization during the etchingprocess and control the etching of the sidewalls of the features.

Therefore, etch selectivity can be managed by controlling formation andremoval of carbon-containing radicals that form polymer deposits on thesubstrate. The absence of hydrogen in the process gas provides increasedamounts of free fluorine radicals for etching the exposed silicon basedmaterial and etching any polymer deposits formed within the featuresbeing etched, which increases the etch selectively to silicon ascompared to the photoresist.

Etch selectivities of greater than one, for example, an etch selectivityof silicon to photoresist of 2:1 or greater, where the etching rate ofsilicon based material is greater than the etching rate of photoresistmaterial, may then be produced by using hydrogen free fluorocarbon gasesin etching processes. With a selectivity greater than one, ananisotropic etch occurs on the silicon based material to form thefeatures of the photomask with minimal defects. Additionally, withminimal formation of passivating deposits or known rates of formation ofpassivating deposits, a known etch rate under different processingconditions can be established and consistent etching can be performed.

Additionally, inert gases added to the process gas form ionizedsputtering species and may further sputter-off any formed polymerdeposits on the sidewalls of the freshly etched features, therebyreducing any passivating deposits and providing a controllable etchrate. It has been observed that the inclusion of an inert gas into theprocessing gas provides better etching uniformity in features to helpproduce features with flat bottoms and smooth sidewalls. Further, theapplication of bias power during the etching process accelerates thespeed and provides more directionality of the etching radicals withrespect to the surface of the substrate, thereby producing a moreanisotropic etching of the silicon based material.

Increased bias levels provide better control of the radicals as theradicals approach the substrate in a path more normal to the surface ofthe substrate than could be achieved in the absence of a bias power.With the path more normal to the surface of the substrate, the processcan produce straighter etching of the sidewalls, particularly thebottoms of the sidewalls, and a more uniform etch of the bottoms of thefeatures, which results in flat, even bottoms than would occurotherwise. Thereby features are etched more precisely and produce highprofile angles formed between the bottom and the sidewall of thefeatures with little or no tapering. It has been observed that highprofile angles with straight side walls and flat bottoms have beenachieved by applying a bias power of about 50 to about 200 watts to thesubstrate during the etching process.

Additionally, it is believed that controlling the processing parametersof the etching process will further provide control over the formationof polymer deposits on the substrate surface. It has been observed thatan increase in source RF power levels will increase the disassociationof the processing gas and increase the formation of polymer radicals inthe plasma. An increase in the formation of polymer radicals results inincreased polymerization and produces greater amounts of polymerdeposits on the substrate. Therefore, reduced power levels, such asbetween about 50 Watts and about 200 watts are used to generatesputtering species while minimizing polymer deposits.

Further, increases in the deposition pressures have been observed toincrease polymerization as more material is available to bedisassociated to form polymer radicals. Therefore, low chamberpressures, such as between about 2 milliTorr and about 50 milliTorr, areused to minimize polymer formation during the etching process. In oneembodiment, a pressure between about 2 milliTorr and about 10 milliTorris used during the etching process.

It is believed that the formation of polymer deposits increase with theincrease in flow rates of fluorocarbon gases as more material isprovided to the plasma process. The increased amount of material in theplasma allows for the increased generation of polymer radicals, whichdeposit as polymers on the substrate surface. A flow rate between about25 sccm and about 100 sccm is used to further minimize polymer formationwhile providing an acceptable commercial etch rate. The addition of aninert gas can dilute the fluorocarbon material in the plasma, therebyproviding less polymer radical formation. The additional inert gas canfurther increase the etching rate by the production of inert gas specieswhich can etch the substrate surface and polymer formations formedthereon.

In another aspect of the invention, a controllable amount of hydrogencan be provided along with the processing gases to facilitate control ofthe etching process and to prevent overetching of features. It iscontemplated that the hydrogen may be introduced by hydrogen containingfluorocarbons with low hydrogen contents, such as C₂HF₅ and CHF₃, or byfree hydrogen gas to enable the formation of a controllable amount ofpolymer deposits to prevent over etching and the loss of the dimensionsof the features. Such hydrogen introduction may be performed during theinitial etch process or may be introduced at the end of the etch processwhen overetching of features is most likely to occur. In either case, aimproved degree of control over deposit formation can be achieved thancan be achieved with either non-hydrogen containing fluorocarbons orhydrogen containing fluorocarbons. The use of hydrogen on a controlledbasis can, in some embodiments, obtain the precision necessary to defineprecisely the structures and dimensions of the photomask's quartzfeatures including alternating phase shift structures.

The invention is further described by the following examples which arenot intended to limit the scope of the claimed invention.

EXAMPLES

For the following examples, SEM photographs of the etched features wereused to measure (i) the quartz etch rate, (ii) the etching selectivityratio of the quartz etching to photoresist etching, (iii) etching rateand etching rate uniformity, and (iv) the profile angles and thestraightness of sidewalls and the flatness of the bottom of thefeatures. Etch rates were calculated by measuring the depth of thefeatures etched in the substrates with respect to the time of theetching process. The etching selectivity ratio was calculated from theratio of the etch rate of the quartz layers to the etch rate of thephotoresist layer. The etch rate uniformity was calculated using atleast 10 measured points. The profile angles were measured byconventionally known methods.

Silicon Plasma Etching:

Example 1 Prior Art Chemistry

The following example was performed with hydrogen containingfluorocarbons. A substrate was prepared by depositing a 1 μm thick layerof silicon oxide on a silicon substrate. A DUV photoresist was thendeposited and etched to expose the underlying substrate. The preparedsubstrate was then introduced into a DPS™ plasma etching chamber. Apre-cleaning step was performed on the substrate to remove processingcontaminants prior to the etching process by introducing oxygen gas at aflow rate of about 30 sccm into the chamber maintained at a chamberpressure of about 10 milliTorr and striking a plasma at about 200 Wattfor about 60 seconds. The substrate was then etched at a chamberpressure of about 10 milliTorr by introducing a processing gascomprising CHF₃, at a flow rate of about 25 sccm, and oxygen, at a flowrate of about 3 sccm, into the chamber and applying a source power tothe coil of about 100 Watts and a bias power to the substrate ofapproximately 200 watts for about 300 seconds. The substrate was thenremoved and SEM photographs of approximately 25 etched features, 10 viasand 15 line patterns, were taken and examined as explained above and theresults are shown in Table 1 below.

Example 2 Silicon Oxide Etching by Hydrogen Free Fluorocarbon Gases

A substrate was prepared by depositing a 1 μm thick layer of siliconoxide on a silicon substrate. A DUV photoresist was then deposited andetched to expose the underlying substrate. The prepared substrate wasthen introduced into a DPS™ plasma etching chamber. A pre-cleaning stepwas performed on the substrate to remove processing contaminants priorto the etching process by introducing oxygen gas at a flow rate of about30 sccm into the processing chamber maintained at a chamber pressure ofabout 10 milliTorr and striking a plasma at about 200 Watt for about 60seconds. The substrate was then etched at a chamber pressure of about 10milliTorr by introducing C₂F₆ into the chamber at a flow rate of about25 sccm and applying a source power to the coil of about 100 Watts and abias power to the substrate of approximately 200 watts for about 300seconds. The substrate was then removed and SEM photographs ofapproximately 25 etched features, 10 vias and 15 line patterns, weretaken and examined as in Example 1. The results of the examination ofthe SEM photographs of the substrates of Examples 1 and 2 are shown inTable 1 below. TABLE 1 Etch Performance for Examples 1 and 2. Example 1:CHF₃/O₂ Example 2: C₂F₆ Profile Angle 85°-88° 87°-89° Etch Rate, Å/min230 270 Resist to Quartz Selectivity 1.4:1 1.3:1 Uniformity 1.0 to 3.0%0.5 to 1.5%

For Examples 1 and 2, the hydrogen free fluorocarbon etching processdescribed herein produced improved etching control with a higher profileangles sharper feature formation, and improved uniformity at a higheretch rate while having a selectivity comparable to the prior artchemistry and processing conditions. Production of Alternating PhaseShift Masks:

Example 3 Etching by Prior Art Chemistry

A substrate was prepared as follows. A chromium layer of approximately100 nm was deposited on a layer of optical quality quartz. A DUVphotoresist was then deposited and patterned on the chromium layer, andthe chromium layer was etched to expose the underlying quartz material.The DUV photoresist was removed and another DUV photoresist layer wasapplied and patterned to expose the underlying quartz material. Theprepared substrate was then introduced into a DPS™ metal etch chamber. Apre-cleaning step was performed on the substrate to remove any errors inphotoresist etching, such as rounding at the bottom of the featuresformed in the photoresist material during the photoresist etchingprocess, prior to etching the quartz layer.

The substrate was etched using a prior art chemistry by introducingCHF₃, at a flow rate of about 25 sccm, and oxygen, at a flow rate ofabout 3 sccm, into the processing chamber maintained at a chamberpressure of about 2 milliTorr, and applying a source power from the coilof about 50 Watts and a bias power to the substrate of about 100 wattsfor about 600 seconds. The substrate was then removed and SEMphotographs of approximately 25 etched features, 10 vias and 15 linepatterns, were taken and examined as explained above and the results areshown in Table 2 below.

Example 4 Etching by Hydrogen Free Fluorocarbon Gases

Example 4 was performed according to the process of Example 3 with themodification of the composition of the processing gas and themodification of the plasma and bias power applied to the processing gas.The substrate was etched by introducing C₂F₆ into the chamber at a flowrate of about 25 sccm to achieve a chamber pressure of about 2 milliTorrand applying a source power to the coil of about 100 Watts and a biaspower to the substrate of about 200 watts for about 600 seconds. Thesubstrate was then removed and SEM photographs of approximately 25etched features, 10 vias and 15 line patterns were taken and examined asexplained above and the results are shown in Table 2 below.

Example 5 Etching by Hydrogen Free Fluorocarbon Gases

Example 5 was performed according to the process of Example 3 with themodification of the plasma and bias power applied to the processing gas.The substrate was etched by introducing C₂F₆ into the chamber at a flowrate of about 25 sccm to achieve a chamber pressure of about 2 milliTorrand applying a source power to the coil of about 50 Watts and a biaspower to the substrate of about 100 watts for about 600 seconds. Afteretching, the substrate was then removed and SEM photographs ofapproximately 25 etched features, 10 vias and 15 line patterns weretaken and examined as explained above. The etching results of Examples3, 4, and 5 are shown in Table 2 below. TABLE 2 Etch Performance forExamples 3, 4, and 5. Ex. 3: CHF₃/O₂ Ex. 4: C₂F₆ Ex. 5: C₂F₆ ProfileAngle 88° 87° 86° Etch Rate, nm/min 27 69 28 Uniformity Error 3.0 1.61.9

For Examples 3, 4, and 5, the hydrogen free fluorocarbon etching processproduced good profile control with high profile angles and sharp featureformation than the prior art chemistry and processing conditions.Additionally, the hydrogen free fluorocarbon etching process has lessuniformity error and exhibited reduced defect density in comparison theCHF₃/O₂ chemistry. The experimental data from Table 2 and theobservations of the SEM photographs of the processed substrates indicatethe hydrogen free fluorocarbon gases produce substrate features withhigh profile angles, straight vertical sidewalls, flat feature bottoms,and a lower defect density.

Example 6 Etching with a Polymerization Limiting Gas

Example 6 was performed according to the process of Example 3 with theaddition of a polymerization limiting compound. The substrate was etchedby introducing C₂F₆ into the chamber at a flow rate of about 25 sccm andoxygen at a flow rate of 3 to 5 sccm to achieve a chamber pressure ofabout 2 milliTorr and applying a source power to the coil of about 50Watts and a bias power to the substrate of about 100 watts for about 600seconds.

While foregoing is directed to the preferred embodiment of theinvention, other and further embodiments of the invention may be devisedwithout departing from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. A method for etching a reticle, comprising: placing the reticle on asupport member in a processing chamber, wherein the reticle comprises anoptically transparent material and is maintained at a temperature ofabout 150° C. or less; introducing a processing gas comprising one ormore hydrogen free fluorocarbons into the processing chamber; supplyinga source of RF power to generate a plasma in the processing chamber; andetching exposed portions of the optically transparent material.
 2. Themethod of claim 1, wherein the optically transparent material isselected from the group of quartz (SiO₂), molybdenum silicide (MoSi),molybdenum silicon oxynitride (MoSiON), and combinations thereof.
 3. Themethod of claim 1, wherein the one or more hydrogen free fluorocarbonshave the formula C_(X)F_(Y), where x is an integer from 1 to 5 and y isan integer from 4 to
 8. 4. The method of claim 1, wherein the one ormore hydrogen free fluorocarbons are selected from the group of CF₄,C₂F₆, C₄F₆, C₃F₈, C₄F₈, C₅F₈, and combinations thereof.
 5. The method ofclaim 1, wherein delivering power to the processing chamber to generatea plasma by supplying a source RF power between about 50 Watts and about200 Watts to a coil and supplying a bias power to the support memberbetween about 50 Watts and about 200 Watts.
 6. The method of claim 1,wherein delivering power to the processing chamber to generate a plasmaby supplying a source RF power of about 500 Watts or less to a coil andsupplying a bias power to the support member of about 500 Watts or less.7. The method of claim 1, wherein the processing chamber is maintainedat a pressure between about 2 milliTorr and about 50 milliTorr.
 8. Themethod of claim 1, wherein the processing gas further comprises an inertgas selected from the group of argon, helium, and combinations thereof.9. The method of claim 1, wherein the processing gas further comprises apolymerization limiting gas selected from the group of oxygen containingcompounds, nitrogen containing compounds, and combinations thereof. 10.The method of claim 1, wherein the method comprises introducing the oneor more hydrogen free fluorocarbon gases into the processing chamber ata flow rate between about 25 sccm and about 100 sccm, introducing theone or more inert gases into the processing chamber at a flow ratebetween about 30 sccm and about 150 sccm, maintaining the processingchamber at a pressure between about 2 milliTorr and about 50 milliTorr,and maintaining the reticle at a temperature between about 50° C. andabout 150° C. while generating a plasma by supplying a RF power betweenabout 50 watts and 200 watts and supplying a bias power to the supportmember between about 50 Watts and about 200 Watts.
 11. A method foretching a reticle comprising optically transparent quartz and apatterned chromium layer disposed thereon, the method comprising:placing the reticle on a support member in a processing chamber having acoil; maintaining the reticle at a temperature of about 150° C. or less;introducing C₂F₆ into the processing chamber at a flow rate betweenabout 25 sccm and about 50 sccm; introducing an inert gas into theprocessing chamber at a flow rate between about 30 sccm and about 50sccm; maintaining a chamber pressure between about 2 milliTorr and about50 milliTorr supplying a source RF power between about 50 watts andabout 200 Watts to a coil; supplying a bias power between about 50 wattsand about 200 Watts to the support member; and etching exposed portionsof the optically transparent quartz.
 12. The method of claim 10, whereinthe processing gas further comprises an inert gas selected from thegroup of argon, helium, and combinations thereof.
 13. The method ofclaim 10, wherein the inert gas may be introduced into the processingchamber at a flow rate between about 30 sccm and about 150 sccm.
 14. Themethod of claim 1, wherein the processing gas further comprises apolymerization limiting gas selected from the group of oxygen containingcompounds, nitrogen containing compounds, and combinations thereof. 15.A method for etching a reticle comprising an optically transparentmaterial, a patterned metal layer deposited on the optically transparentmaterial, and a patterned resist material deposited on the patternedmetal layer, the method comprising: placing the reticle on a supportmember in a processing chamber; introducing a processing gas comprising:one or more hydrogen free fluorocarbon gases; one or more inert gases;one or more polymerization limiting gases; delivering power to theprocessing chamber by supplying a source RF power of about 500 Watts orless to a coil and supplying a bias power to the support member of about500 Watts or less to generate a plasma; and etching exposed portions ofthe optically transparent material.
 16. The method of claim 15, whereinthe optically transparent material is selected from the group of quartz(SiO₂), molybdenum silicide (MoSi), molybdenum silicon oxynitride(MoSiON), and combinations thereof.
 17. The method of claim 15, whereinthe one or more hydrogen free fluorocarbon gases have the formulaC_(X)F_(Y), where x is an integer from 1 to 5 and y is an integer from 4to
 8. 18. The method of claim 17, wherein the one or more hydrogen freefluorocarbon gases are selected from the group of CF₄, C₂F₆, C₄F₆, C₃F₈,C₄F₈, C₅F₈, and combinations thereof.
 19. The method of claim 15,wherein the one or more inert gases are selected from the group ofargon, helium, and combinations thereof.
 20. The method of claim 15,wherein the one or more polymerization limiting gases are selected fromthe group of oxygen containing compounds, nitrogen containing compounds,and combinations thereof.
 21. The method of claim 15, wherein the one ormore hydrogen-free fluorocarbon gases are introduced into the chamber ata flow rate between about 25 sccm and about 50 sccm, the one or moreinert gases are introduced into the processing chamber at a flow ratebetween about 30 sccm and about 50 sccm, and the one or morepolymerization limiting gases are introduced into the processing chamberat a flow rate between about 1 sccm and about 10 sccm.
 22. The method ofclaim 15, wherein the processing chamber is maintained at a pressurebetween about 2 milliTorr and about 50 milliTorr.
 23. The method ofclaim 15, wherein the method comprises introducing the one or morehydrogen free fluorocarbon gases into the processing chamber at a flowrate between about 25 sccm and about 100 sccm, introducing the one ormore inert gases into the processing chamber at a flow rate betweenabout 30 sccm and about 150 sccm, introducing the one or morepolymerization limiting gases into the processing chamber at a flow ratebetween about 1 sccm and about 10 sccm, maintaining the processingchamber at a pressure between about 2 milliTorr and about 50 milliTorr,and maintaining the reticle at a temperature between about 50° C. andabout 150° C. while generating a plasma by supplying a RF power betweenbetween about 50 Watts and about 200 Watts.